The apparatus of the invention is particularly useful with devices known as "smart two-wire process transmitters", however, the principles can be used with numerous other devices that must be set up or configured in one of a multitude of different modes depending on the application.
Devices known as process signal transmitters accept one form of various low level signals from a sensor or transducer and convert such signals to a standard form known as a 4 to 20 mA current. Such signal transmitters are used in the chemical and process industries and are used when measuring temperature, pressure, flow, level and the various other process variables.
Generally to measure each of these different process variables a different design transmitter is required. Transmitters that are designed using classical analog circuit technology even require different models of transmitters depending on the range of the measured variable.
The purpose of such a transmitter is to take the low level (microvolt and millivolt level) signals available from thermocouples, strain gages and other sensors; convert these low level signals near the location of the sensor to a suitable high level signal, and then with a much higher degree of noise immunity, transmit such higher level signals to some desired remote location within the same plant, such as the control room. The low level signals directly out of the sensor are easily affected by electromagnetic interference, signal loss due to line resistance, ground loop interference and various other causes, thus degrading the accuracy of measurement. The 4 to 20 mA current signal has been estabilshed as an industry standard because analog signals transmitted in this fashion are much less affected by other interfering signals, thus helping to preserve the measurement accuracy.
The term "4 to 20 mA loop" refers to an analog signal transmission standard adopted by the process industry. According to this standard, an analog signal to be transmitted some distance, usually within the confines of a manufacturing plant, is converted to a current signal in the range of 4 mA and 20 mA. A certain value of the measured parameter can be made to correspond to the 4 mA value of the output. This is usually referred to as the LRV (Lower Range Value) or "zero". A certain other value of the measured parameter is then made to correspond to the 20 mA value of the output. This is them referred to as the URV (Upper Range Value). The difference in inpt signals between the URV and the LRV is referred to as the "span". The output current will then vary between the 4 and 20 mA values as the measured parameter changes between its LRV and URV values.
For the past several decades, two-wire process signal transmitters have used analog circuit technology implemented using transistors and later various integrated circuit operational amplifiers, OPAMP. Numerous small improvements to these analog transmitters have been developed over the years, however, the analog signal transmitter technology is now considered to be mature. Improvements are generally of the nature of refinements of the existing technology, rather than solutions to major unresolved problems. Process signal transmitters using "analog only" technology continue to have major performance shortcomings which significantly affect their measurement accuracy and ease of use.
In recent years some companies have introduced "Smart" or "Intelligent" two wire process transmitters.
All of these smart two wire transmitters are distinguished from their analog predecessors by utilizing one or more microprocessors for their operation. The present invention includes a technologically advanced smart temperature transmitter which provides a major step improvement in performance, and many new and desired features which could not be achieved in earlier analog transmitters.
Where there is a microprocessor as part of a system, like it is in a smart two wire transmitter, implementing digital communications with another data terminal or computer is a relatively simple matter. The microprocessor has the facility to handle all of the digital data communications tasks required. For this reason all of the smart two wire transmitters offer some form of digital communications with either a hand held terminal or a process computer (a computer used to control a chemical process). Such digital communications in a smart two wire tranmsitter are used to select the desired sensor, or to select the desired zero and span for the output, or to set the open sensor alarm state, and to perform other similar set up functions.
Smart transmitters offer a wide range of versatility and, therefore, require to be "told" much more information to set them up or configure them in the desired mode. The following is a typical list of decisions the user needs to make and somehow convey to the transmitter to properly configure it prior to use: